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A finite superposition of energy eigenrays: exact phases, laws and return times
A finite, exact model computation for a declared isolated system: the evolution of a nonzero finite superposition of orthogonal energy eigenrays, the resulting trigonometric-polynomial form of the outcome laws of bounded assigned sharp observables, the exact return-time criterion for the ray, the two-energy period, and the stationary case. The eigenvalues are model parameters; the item asserts no apparatus, no detection model and no empirical value, and no finite set of energies is identified with a physical spectrum.
Example
Let be the state space of a declared isolated system of the framework (The state space of the framework: preparations, pure rays and density operators), let be its assigned energy observable with spectral projection-valued measure and flow (Assigned energy observable, unitary flow and the two pictures of autonomous evolution), and let be a finite orthonormal family of eigenvectors of with real eigenvalues : for . Fix coefficients and the nonzero vector For a nonzero vector , write for the outcome law of the pure preparation , computed on the unit representative as in Assigned sharp observables and the outcome law of the framework clause (b) and State vectors, unit vectors, rays, normalization and phases of a Hilbert space. Then, with :
(i) for every , and ; if has one element, or more generally if all with coincide, the ray is constant.
(ii) For every bounded assigned sharp observable and every Borel set , the outcome law in the preparation is Thus the probability is a finite trigonometric polynomial in with frequencies .
(iii) For the following are equivalent: (a) for every ; (b) is independent of ; (c) for all .
(iv) Consequently a positive return time exists if and only if there are and integers () with for all , where is any fixed element of ; and then is a return time.
(v) Two-energy case , , : the positive return times are exactly , ; the least return time is , which is the period of the precession example with .
(vi) In the stationary case of (i), every outcome law of every assigned sharp observable is independent of : .
The computation is a model-internal one: the eigenvalues are parameters of the assigned observable, and no identification with a physical spectrum, apparatus or measured value is asserted.
Facts & Assumptions
Given: A nonzero complex Hilbert space ; the assigned energy observable with flow ; a finite orthonormal family of eigenvectors with real eigenvalues ; coefficients and the nonzero vector ; ; the evolved vectors ; a bounded assigned sharp observable with spectral measure ; a Borel set ; the Axiom of Countable Choice and the Axiom of Choice, the choice hypotheses of the cited spectral suppliers (The Axiom of Countable Choice (), The Axiom of Choice).
Eigenrays evolve by phases. If with , then for every , and the flow is linear with (Energy conservation, conserved sharp observables and the stationary eigenrays clause (c), The Schroedinger-picture flow: unitarity, domain invariance and the domain-qualified equation clause (a), Conserved spectral probability laws for commuting observables and stationary eigenrays clause (d)).
Outcome law for a pure preparation. for a unit vector, and the number depends only on the ray ; the inner product is linear in the first argument and conjugate-linear in the second (Assigned sharp observables and the outcome law of the framework clause (b), State vectors, unit vectors, rays, normalization and phases of a Hilbert space, Real and complex inner product spaces, with the inner product linear in the first argument).
Complex exponential algebra. For real : , and ; for complex , (Euler's formula: for every real , , and the complex exponential extends the real exponential, , , and , Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive). Moreover , so for real one has exactly for (, and exactly when ).
Orthonormality. , and every as above is nonzero exactly when is nonempty (Real and complex inner product spaces, with the inner product linear in the first argument, The induced length is a norm).
Verification
[Evolution of the superposition] By [F1] applied to each eigenvector, ; linearity of gives , and by the norm statement of [F1]. If all with equal a common , then , so for every . This is (i).
[Trigonometric-polynomial law] Assume first . By [F2] and step 1.1, using conjugate-linearity in the second argument, . For general nonzero , the unit representative has coefficients by step 1.1; substituting these into the unit-vector calculation divides the displayed sum by , as asserted.
[Return-time criterion] Let . If (a) holds, then for every there is with ; pairing with for and using orthonormality [F4] gives , hence for every , which is (b). If (b) holds with common value , then by step 1.1, which is (a). Finally (b) holds exactly when for all , and by the exponential kernel of [F3] this is exactly the integrality condition (c). This proves (iii).
[Common frequency and return time] Condition (iv) is the statement that the numbers are integer multiples of one positive number ; by step 2.2 with and [F3], for all , so is a return time. Conversely, if is a return time, then by step 2.2(c) applied with we have for all , that is with and integers ; this proves the equivalence of (iv).
[Two energies] Let , , and . By step 2.2(c) with the return times are exactly the solutions of , that is with , ; the least such is for . This is (v).
[Stationary case] If all , , equal a common , then by step 1.1 and ; by [F2] the outcome law of every bounded assigned sharp observable depends only on the ray, so for every . This is (vi), and it agrees with the stationary-state characterization of Energy conservation, conserved sharp observables and the stationary eigenrays clause (c).
Steps 1.1, 2.1, 2.2, 3.1, 3.2 and 3.3 establish (i)-(vi) for the explicit model data.
Remarks
- The relative phases are physical. Only the relative phases appear in the laws; a common phase of would not change any outcome law, in agreement with the ray reading of pure preparations.
- Existence of a common frequency is a number-theoretic condition on the model data. For two distinct energies it always holds, and the least return time is the inverse Bohr frequency of the precession example. For three or more energies it may fail, in which case the ray never returns to itself, although the laws remain finite sums of oscillations with the Bohr frequencies.
- No detector is modeled. Exactly as on the precession example, the probabilities displayed are model outcome laws; no measurement interaction, finite resolution, efficiency or record is introduced.
Depends on
, , and
The induced length is a norm
The Axiom of Choice
The Axiom of Countable Choice ()
Hilbert space
Real and complex inner product spaces, with the inner product linear in the first argument
Assigned energy observable, unitary flow and the two pictures of autonomous evolution
Dimensions, SI unit conventions and quantity types of the framework
Assigned sharp observables and the outcome law of the framework
The state space of the framework: preparations, pure rays and density operators
The state space: density operators, pure and mixed states, and purity
State vectors, unit vectors, rays, normalization and phases of a Hilbert space
Projection valued measure
Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive
The Schroedinger-picture flow: unitarity, domain invariance and the domain-qualified equation
Energy conservation, conserved sharp observables and the stationary eigenrays
, and the complex exponential extends the real exponential
Euler's formula: for every real
, and exactly when
Conserved spectral probability laws for commuting observables and stationary eigenrays
Spectral theorem for unbounded self-adjoint operators (PVM form)
Dependency tree · 2 levels
Sources
- J. Binney and D. Skinner, The Physics of Quantum Mechanics, Section 2.2, printed pages 21-23 (equation (2.32): the evolution of a general state as a superposition of energy eigenstates with relative phases e^{-iE_n t/hbar})
- B. Zwiebach, MIT 8.05 Quantum Physics II (Fall 2013), Chapter 6 'Quantum Dynamics', Section 2.3, printed pages 15-17 (the evolution operator for a time-independent Hamiltonian and its application to eigenstates)